Heterophase fluctuations in supercooled liquids and polymers

Glass-forming liquids and polymers consist of 'solid-like' and 'fluid-like' clusters of a finite lifetime in a dynamic equilibrium. The experimental observations leading to the heterophase fluctuation model are reviewed. They are: (i) long range density fluctuations as measured by static light scattering and ultra small angle X-ray scattering, (ii) ultra- slow hydrodynamic modes of density fluctuations as observed by photon correlation spectroscopy, (iii) temperature depending structural changes as studied by wide angle X-ray scattering. In order to explain these observations a new order parameter n(s)(x, t) is proposed, which describes the local mass fraction of energetically preferred structures ('aperiodic solid states'). The thermodynamic potentials can be analyzed in terms of the temperature dependence of <n(s)(T)>. The alpha- relaxation process of supercooled liquids is directly related to the local fluctuations of n(s)(x, t). (C) 2002 Elsevier Science B.V. All rights reserved